The present invention relates to a device for sensing oxygen concentration in a heated gaseous mixture such as an engine exhaust gas. The device is fundamentally based on a solid oxygen-ion electrolyte concentration cell.
In recent years it has become of great importance to measure oxygen concentrations in exhaust gases from various engines, particularly from automotive internal combustion engines, mainly for the following reason.
In various systems now in practice and/or under development for converting harmful or noxious components of an engine exhaust gas such as unburned hydrocarbons, carbon monoxide and oxides of nitrogen into harmless substances such as water, carbon dioxide and nitrogen, the air-to-fuel ratio of a combustible mixture fed to the engine is frequently an important factor for the operation of such systems. For example, the air-to-fuel ratio must be maintained practically stoichiometric in order to use a new catalyst, which is expected to cause all or part of the above-mentioned harmful substances to undergo catalytic reactions to give the above-mentioned harmless substances at high efficiencies in practical systems. It is known that the air-to-fuel ratio can be estimated from the partial pressure of oxygen in the exhaust gas when the air-to-fuel ratio is around the stoichiometric ratio. When the exhaust gas is subjected to no chemical treatment, the oxygen partial pressure in the exhaust gas is usually of the order of 10.sup.-2 atm for an air-to-fuel ratio higher than the stoichiometric ratio but of the order of 10.sup.-3 atm for an air-to-fuel ratio lower than the stoichiometric. When, however, the exhaust gas is subjected to either an after-burning or catalytic reactions and is in thermodynamically equilibrium states, the oxygen partial pressure exhibits a sharp change at the stoichiometric air-to-fuel ratio of the combustible mixture from a value of the order of 10.sup.-3 atm for an air-rich ratio to a value of the order of 10.sup.-15 to 10.sup.-30 atm for a fuel-rich ratio. Therefore, it is possible to detect whether the air-to-fuel ratio for an engine in operation is maintained at the stoichiometric ratio or deviated therefrom by measuring the oxygen partial pressure in the exhaust gas. A signal representing the measured oxygen partial pressure or the estimated air-to-fuel ratio may be supplied to a control system for controlling, for example, the rate of fuel injection into the engine in order to maintain the air-to-fuel ratio at a predetermined or the stoichiometric ratio thereby to accomplish the modification of the exhaust gas at expected efficiencies.
A typical sensor now in practice for sensing the oxygen partial pressure in a heated gaseous mixture such as an engine exhaust gas is principally based on a concentration cell made of a layer of a solid electrolyte in which oxygen ions function as the electron carriers. The magnitude of the electromotive force in this type of cell is dependent on the ratio of an oxygen partial pressure in an atmosphere communicating with one side of the electrolyte layer and another oxygen partial pressure applied to the other side and determined by the well known Nernst's equation.
In practical applications of this oxygen sensor to engine exhaust systems, the electrolyte layer must be shaped in such a manner that one side thereof is exposed to an exhaust gas flowing in the exhaust system while the other side is exposed to the atmospheric air which is usually used as a reference gas.